The present invention relates to the protection of conductive surfaces in electrical circuits and, in particular, to the protection of conductive surfaces in a thermal inkjet print head circuit.
Generally, an inkjet image is formed when a precise pattern of dots is ejected from a drop generating device known as a xe2x80x9cprint headxe2x80x9d onto a printing medium. The typical inkjet print head has an array of precisely formed nozzles in an orifice plate that is attached to an ink barrier layer on a thermal inkjet print head substrate. FIG. 1 shows a print head substrate 12 with a single nozzle 10A formed in an orifice plate 10. The substrate incorporates an array of firing chambers that receive liquid ink from an ink reservoir. Referring to FIG. 1, each chamber 11 has a thin-film resistor 13, known as a xe2x80x9cfiring resistorxe2x80x9d, located opposite each nozzle 10A so ink can collect between the firing resistor and the nozzle. When electric printing pulses heat the thermal inkjet firing resistor, a small volume of ink adjacent the firing resistor is heated, vaporizing a bubble of ink, and thereby ejecting a drop of ink from the print head. The droplets strike the printing medium and then dry to form xe2x80x9cdotsxe2x80x9d that, when viewed together, form the printed image.
Control signals for causing the resistors to xe2x80x9cfirexe2x80x9d originate from the printers processing unit. An electrical connection is established with the thin film resistor 13 by lithographically patterned conductive traces (not shown) connected between the resistor and an exposed bonding pad 14 remotely located from the resistor. The bonding pad facilitates connection with an exposed conductive lead on a flexible tape circuit 15 (also referred to as a flex circuit) that is carried on the print head. The flex circuit conveys control or xe2x80x9cfiringxe2x80x9d signals from the printer""s processor to the resistor.
The flex circuit is particularly suited for connecting electrical components where one or both connections may be coupled to moving parts. For example, in the case of an inkjet printer, the print head, which is scanned across the print media while ejecting droplets of ink, is electrically connected to the printer""s processor with a flex circuit. In general, flex circuits are fine, conductive filaments or formed traces laminated between, overmolded with, or otherwise adhered to, a layer of a flexible, dielectric material, such as a polyimide tape. The interconnect circuit so formed can be bent or looped without affecting the electrical interconnections between the electrical components it connects.
A connection between a given firing resistor 13 and a conductive trace on the flex circuit 15 is created by attaching a conductive bonding beam 16 from the bonding pad 14 to the trace. After bonding, exposed conductive connection areas including at least the bonding pads, bonding beams and conductive traces on the flex circuit are protected/covered from the corrosive environment of the print head.
A previous approach for protecting the exposed conductive connection uses two different coating techniques to accommodate the surface characteristics of two distinct areas of the exposed connection. Specifically, since a first area including the bonding pad 14 and the bonding beam 16 is characterized by small crevices and irregularly shaped surfaces, this area is protected by dispensing a drop of protective coating 17 (including the pad and beam) thereby filling-in small crevices that are hard to coat and covering the entire area. On the other hand, a second area including the exposed conductive traces on the flex circuit 15 is characteristically planer. Consequently, this area is more adapted to a process in which the protective coating is laminated over the exposed traces using, for instance, a screen coating, or sheet/curtain process, thereby forming a protective layer of lamination 18 over the area.
One disadvantage of this coating technique is that due to the irregularity of the surface of the first area, the dispensed coating may have small unprotected pin holes or bubbles and may be unevenly distributedxe2x80x94resulting in unreliable protection of this area. In addition, covering the first and second areas using different processing techniques results in additional processing steps. Moreover, gaps of protection can occur between the dispensed protective coating 17 and the laminated coating 18.
What would be desirable is a reliable method of protecting the exposed connection areas between a print head and a flex circuit or, in general, between a device and a flex circuit using a single coating process.
A method of processing and a structure formed thereof. The method is performed on an exposed conductive connection area between a first device and a flexible tape circuit by coupling the exposed connection to a first voltage and immersing the exposed connection in an electrophoretic solution in contact with an electrode at a second voltage potential thereby establishing a current between the electrode and the exposed connection such that the exposed connection is coated with a thin insulating film of uniform thickness by electrophoretic plating.
In a second embodiment of the method of processing and a structure formed thereof, the method is performed on an exposed conductive connection between a thermal inkjet device and a flexible tape circuit connectable to control signals for driving the thermal inkjet device. The method is performed by coupling the exposed connection to a first voltage and immersing the exposed connection in an electrophoretic solution in contact with an electrode at a second voltage potential thereby establishing a current between the electrode and the exposed connection such that the exposed connection is coated with a thin insulating film of uniform thickness by electrophoretic plating.